Input impedance of mosfet buffer?

[Eirik]

I'm on my serch for the ultimate DIY booster but I'm not much into the maths of electronical circuits (some reading has to be done i guess :oops:  ). However, reading Jack Ormans basic buffers,  http://www.muzique.com/lab/buffers.htm he mentions the possibility to add a 10M resistor (between the gate and drain as I understand it?) in the mosfet circuit? Will this affect the input impedance in any way? And what would the input impedance be?

[BJF]

Hi,

With a MOS Fet buffer, you'd need to provide a bias and this can be done with two 10 M resistors, one to ground and one to B+, and source would sit at halfpotential.

The resulting inputimpedance would be 10M//10M// IZ T1 and as the mos fet would have on order of 100MOhm's
input impedance, final IZ would be about 5 Mega Ohms.

You might want to check what loading your pick up would like to see to soften the resonant peak of it if this is to be a pick up booster.
5 Mega Ohms would most likely be in excess and should not present you with any trouble.

To properly determine what imputimpedance of a buffer should desirably be
one would first examine the source as in is it a guitar pick up or other transducer.

You can in this get a  5 Mega Ohm pot and hook up as reostat and dial in the response you want from in thhis case now 2,5Mega to zero.
Happy medium could be around a couple of houndred Kilo Ohms.

Regards
BJ

[Eirik]

Thanks! I love this Forum!

Eirik